feat(kanban): Persist categories to .automaker/categories.json

Categories are now saved to a categories.json file in the .automaker
directory. This ensures categories persist even when all feature cards
are deleted. The category autocomplete now shows suggestions from both
existing features AND the persisted categories file.

- Add persistedCategories state and load/save functions in board-view
- Merge persisted categories with feature categories for suggestions
- Auto-save categories when adding or editing features
- Update mock Electron API to handle categories.json file

Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/src/lib/electron.ts

@@ -152,6 +152,10 @@ export const getElectronAPI = (): ElectronAPI => {
       if (filePath.endsWith("feature_list.json")) {
         return { success: true, content: JSON.stringify(mockFeatures, null, 2) };
       }
+      if (filePath.endsWith("categories.json")) {
+        // Return empty array for categories when file doesn't exist yet
+        return { success: true, content: "[]" };
+      }
       if (filePath.endsWith("app_spec.txt")) {
         return {
           success: true,